Confirming Impurity Effect in Silver-Point Realization from Cell-to-Cell Comparisons

Widiatmo, J. V.; Harada, Koh; Yamazawa, K.; Tamba, J.; Arai, M.

doi:10.1007/s10765-011-1097-5

Cited by 6 publications

(5 citation statements)

References 7 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…For this zinc fixed-point cell, under all circumstances the agreement between the model and experiment improves as the distribution coefficient k → 0. This is consistent with other analyses which show that Raoult's law can be used to describe the effect of impurities on the freezing curve [7][8][9][10][11]. It was shown that the departure from the shape predicted by the numerical model towards the end of freezing could be explained by the fact that a significant amount of metal continues freezing elsewhere in the fixedpoint cell, after the metal immediately surrounding the SPRT sensing element has completely frozen.…”

Section: Discussionsupporting

confidence: 89%

“…The limitations of the analysis and knowledge of liquidus slopes, and whether the analysed sample is representative of the ingot in the fixed-point cell, make the SIE challenging to implement reliably in practice, and other methods are needed to provide supplementary evidence. Analysis of a series of measurements by Yamazawa et al [7][8][9][10] found that in most cases for Sn, Zn, Al and Ag fixed points, the corrections obtained from the freezing curve slope (assuming Raoult's law), SIE and direct cell comparison agreed well, even when the GDMS analysis indicated the presence of high k impurities, although in one case the SIE underestimated the magnitude of the correction relative to the other two methods by 30% [7,11]. Clearly Raoult's law, and other more general analyses of the freezing curve shape where k is not necessarily zero, remains useful.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Optimization of SPRT measurements of freezing in a zinc fixed-point cell

Pearce¹,

Veltcheva²,

Lowe³

et al. 2012

Metrologia

View full text Add to dashboard Cite

A numerical model of solute and heat transport in extremely pure materials is described. Its purpose is to characterize the effect of impurities on the freezing curves of metals containing impurities at the level of less than 1 part per million. It is used to simulate experiments performed using a commercially available zinc fixed-point cell for SPRT calibrations. The aim is to determine the effect of different vertical temperature gradients on the freezing curve and to find out whether a range of conditions could be determined where there was a good fit between theory and experiment. For this fixed-point cell, agreement between the model and experiment improves as the distribution coefficient k → 0. It is found that the model only agrees with the measured freezing curves over the entire freeze for a narrow range of furnace settings where the temperature profile is most uniform. We suggest that this is because if the furnace settings are not optimized, the solid does not grow uniformly, and freezing may continue in regions remote from the SPRT after the material in the vicinity of the SPRT has finished freezing, so distorting the freezing curve. This effect is not present in the model and so the method presented here enables optimization of the furnace to ensure the SPRT is surrounded by a liquid–solid interface over the entire freezing range. We find that the optimum thermal environment is extremely sensitive to the furnace settings; the optimum thermal environment is found when the temperature is slightly cooler at the top of the cell, as measured in the re-entrant well of the cell. We note that optimizing the freezing process is a necessary step towards using a thermal analysis to correct for the effects of impurities in the sample.

show abstract

Section: Discussionsupporting

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Optimization of SPRT measurements of freezing in a zinc fixed-point cell

Pearce¹,

Veltcheva²,

Lowe³

et al. 2012

Metrologia

View full text Add to dashboard Cite

show abstract

“…The detected Co has the same value as the detection limit, so it could be less reliable. Ni was also found in our silver ingot [10] taken out from its crucible after being used so many times. This may reflect an imperfection of our cell fabrication facilities.…”

Section: In the Form Of Error Bars Showingsupporting

confidence: 52%

Study on the Realization of Zinc Point and the Zinc-Point Cell Comparison

et al. 2010

Self Cite

View full text Add to dashboard Cite

Continuing our study on aluminum, tin, and silver points, a study on the realization of the zinc point was conducted. Zinc-point cells were newly fabricated using 6N-nominal grade zinc samples, impurity elements of which were analyzed extensively based on glow-discharge mass spectrometry (GDMS). The present paper reports the temperature measurements done using the newly fabricated cells during the zinc freezing process, under which the zinc fixed point is defined, and the analysis of the freezing curve obtained. Comparisons of zinc-point temperatures realized by the newly fabricated cells (cell-to-cell comparisons) were also conducted. Zinc-point depression due to impurity elements was calculated based on the sum of individual estimates and the impurity element analysis. One of the cells evaluated was drawn out from its crucible and analyzed by GDMS at four points, namely, at around the center of the top, of the middle, of the bottom, and around the outer part of the middle area. The purpose of this cell disassembly is to see whether or not there has been some difference before and after cell fabrication, as well as difference in impurity element distribution within the ingot. From the aforementioned studies, some findings were obtained. First finding is that the homogeneity of the zinc ingot was within 30 %, except for Pb, which was more concentrated in the center part. Second finding is that the cell-to-cell temperature difference changes along with the progressing solidification process. As a consequence, for an accurate cell-to-cell comparison, the locus in the freezing plateau where the comparison is done should be determined. Third finding is that the slope analysis estimates accurately the cell-to-cell comparison, and is consistent with the impurity analysis. This shows that the slope analysis gives extensive information about the effect of impurity to the zinc-point realization, especially after the cell fabrication.

show abstract

“…Considerable work has been performed on the effect of impurities in ITS-90 fixed points, utilising various correction methods on individual cells (Sn [4,5], Zn [6][7][8][9], Al [10] and Ag [11]) and in fixed point cells in general [12][13][14][15][16]. Two key methods have emerged, and are now recommended by the CCT, which are both based on a chemical assay of the metal to determine which impurities are present, and in what quantity.…”

Section: Introductionmentioning

confidence: 99%

A systematic evaluation of contemporary impurity correction methods in ITS-90 aluminium fixed point cells

2017

View full text Add to dashboard Cite

The fixed points of the International Temperature Scale of 1990 (ITS-90) are the basis of the calibration of standard platinum resistance thermometers (SPRTs). Impurities in the fixed point material at the level of parts per million can give rise to an elevation or depression of the fixed point temperature of order of millikelvins, which often represents the most significant contribution to the uncertainty of SPRT calibrations. A number of methods for correcting for the effect of impurities have been advocated, but it is becoming increasingly evident that no single method can be used in isolation. In this investigation, a suite of five aluminium fixed point cells (defined ITS-90 freezing temperature 660.323 °C) have been constructed, each cell using metal sourced from a different supplier. The five cells have very different levels and types of impurities. For each cell, chemical assays based on the glow discharge mass spectroscopy (GDMS) technique have been obtained from three separate laboratories. In addition a series of high quality, long duration freezing curves have been obtained for each cell, using three different high quality SPRTs, all measured under nominally identical conditions. The set of GDMS analyses and freezing curves were then used to compare the different proposed impurity correction methods. It was found that the most consistent corrections were obtained with a hybrid correction method based on the sum of individual estimates (SIE) and overall maximum estimate (OME), namely the SIE/Modified-OME method. Also highly consistent was the correction technique based on fitting a Scheil solidification model to the measured freezing curves, provided certain well defined constraints are applied. Importantly, the most consistent methods are those which do not depend significantly on the chemical assay.

show abstract

Confirming Impurity Effect in Silver-Point Realization from Cell-to-Cell Comparisons

Cited by 6 publications

References 7 publications

Optimization of SPRT measurements of freezing in a zinc fixed-point cell

Optimization of SPRT measurements of freezing in a zinc fixed-point cell

Study on the Realization of Zinc Point and the Zinc-Point Cell Comparison

A systematic evaluation of contemporary impurity correction methods in ITS-90 aluminium fixed point cells

Contact Info

Product

Resources

About